1. Field of the Invention
The present invention relates to a current transformer disposed in a three-phase gas insulated switch device or gas insulated bus, or the like, for detecting a current that flows in a three-phase conductor, and more particularly to a three-phase current transformer in which three air-core coils are received in a pressure vessel together.
2. Description of the Related Art
Conventionally, current transformers using analog signals as a secondary information transmission signal have been widely employed. In current transformers of this type, a current value of a main circuit is detected by using an iron core coil, and the current value thus detected is outputted to the secondary system. Also, an iron core having a large sectional area is employed as the iron core coil. This is because some degree of electric power needs to be ensured in order to operate an electromagnetic relay or the like, and in order prevent deterioration in detection precision due to magnetic saturation.
However, in recent years, digitalization of secondary information transmission signals has advanced. In a current transformer that outputs a digital signal, since only a small amount of electric power needs to be ensured, an air-core coil such as a Rogowskii coil having a small sectional area and no magnetic saturation is used.
Since the iron core used for the iron core coil is made of a ferromagnetic material, a magnetic flux developed by a current that flows in each phase is absorbed into the iron core. For that reason, in the case where the iron core coil is used for a three-phase package type current transformer, the magnetic influence on the coils of other phases is slight. However, because air-core coils such as Rogowskii coils are made of non-magnetic materials such as a high polymer member, the magnetic flux developed by the current of each phase reaches other phases, and the magnetic influence imparted to the coils of the other phases is large. Therefore, in three-phase package type current transformers using air-core coils, a magnetic shield must be provided between the respective phases.
FIG. 8 is a cross-sectional view of a conventional three-phase package type current transformer disclosed in, for example, Japanese Patent Laid-open No. Hei 10-185961. In the figure, three conductors 2a to 2c are received in a cylindrical pressure vessel 1 that is filled with SF6 gas, an insulating and arc-extinguishing gas. Three-phase conductor is comprised of the conductors 2a to 2c. The conductors 2a to 2c are surrounded by first to third Rogowskii coils 3a to 3c which are air-core coils. The first to third Rogowskii coils 3a to 3c are current sensors that obtain current signals proportional to the values of currents that flow in the conductors 2a to 2c. A plurality of magnetic shields 4 for moderating the influence of the magnetic field from other phases are disposed between the respective conductors 2a to 2c. 
In the conventional three-phase package type current transformer thus structured, because the first to third Rogowskii coils 3a to 3c are individually fixed in the pressure vessel 1, the structure for supporting the first to third Rogowskii coils 3a to 3c becomes complicated.
The present invention has been made to solve the above-described problems, and therefore an object of the present invention is to provide a three-phase package type current transformer that can simplify the structure for supporting the air-core coils in the interior of the pressure vessel.
To this end, according to one aspect of the present invention, there is provided a three-phase package type current transformer that is received in a pressure vessel which is filled with an insulating gas, for detecting a current that flows in a three-phase conductor having at least three conductors and arranged within the pressure vessel; the current transformer comprising: a metal adaptor fixed in the interior of the pressure vessel, and having first to third conductor through-holes through which the conductors are inserted and first to third annular coil insertion slots formed in the periphery of the first to third conductor through-holes; and first to third air-core coils disposed in the first to third coil insertion slots; wherein the metal adaptor has a ground potential and serves as a magnetic shield.
According to another aspect of the present invention, there is provided a three-phase package type current transformer that is received in a pressure vessel which is filled with an insulating gas, for detecting a current that flows in a three-phase conductor having at least three conductors and arranged within the pressure vessel; the current transformer comprising: a plate-like insulating spacer having first to third conductor through-holes through which the conductors are inserted, first to third annular spacer slots formed in the periphery of the first to third conductor through-holes, fixed in the interior of the pressure vessel, and supporting the three-phase conductor while electrically insulating the three-phase conductor with respect to the pressure vessel; and first to third air-core coils disposed in the first to third spacer slots.